Quickness of understanding is a mental faculty, but right doing requires the practice of a lifetimeGoethe

I was sitting alone in the restaurant – which serves informal French cuisine with a good selection of regional wines – immersed in the New York Review of Books, when two young men – mid-twenties, one starting out in finance the other in politics – sat at the table next to me. Their discussion was brisk and uninhibited, hard to ignore despite my best intentions. After several minutes of gossip about mutual acquaintances, one changed the subject abruptly.

"Have you read this book by Daniel Kahneman?" he asked, "it's called Thinking Fast and Slow." "No. Is it any good?" "Not really. It's far too long. The basic idea is obvious could have been summarised in 25-pages. He keeps repeating himself and includes endless stories and anecdotes." "OK that's good to know, I won't waste my time reading it."

Momentarily, I was tempted to interrupt their conversation, to explain to them that if they were both to take a week off work, carefully to study Kahneman's book and the wider literature he describes, it would be an investment that would repay them multiple times over the course of their lives. I resisted the temptation and now I fear that both their careers will forever be blighted by loss.

These two men –still young but already in too much of a hurry - were thinking fast rather than thinking slow; which is to say, they were not really thinking at all. When we apply our minds without reflection, without checking carefully for bias, for lack of relevant information in sufficient quantity, and for over-confidence, we tend to apply rules-of-thumb to cases for which they might not be applicable; it is quick but lazy. Some people like to call this "intuition", but I think "prejudice" is the better term.

Heuristic tools, which give standard answers in response to standard questions, are useful when the problems we face are minor and quotidian. However, when things start to get harder, the quick responses soon become inadequate and a more considered approach is required. And when we encounter problems about the most important questions in life – of freedom and duty, of value and meaning, of friendship and happiness - thinking fast is wholly unreliable.

Thinking slow – which is to say, really thinking - takes time and energy, which for evolutionary reasons we are disposed not to want to expend, even when we recognise that the standard answers will not work in this instance. Nonetheless, without investing in the skills and disciplines of careful, reflective thought, we are condemned to rely on the first idea that lodges in our mind, which is often someone else's fast thought, circulating around society like a virus, which we have picked up unknowingly simply through our proximity to those already infected.

What to do about lazy thinking? Hard work seems to be the right answer.

I conceive of slow thought – that is, careful, reflective, unprejudiced thought – as a skill, which we should spend our lifetime acquiring, exercising and improving. When we are young, we are often impatient to learn, and we have the capacity to pick up skills and ideas with great speed. But learning to think is harder than learning to play chess or solve Sudoku puzzles. It is harder than learning to play the piano.

Listening to Glenn Gould's recording of the Goldberg Variations from 1955, when he was 22 years old, and comparing it to the recording he made in 1981, the year before he died aged 50, it seems to me that when he was young he knew how to play Bach well, but when he was older he had also learned how to interpret Bach well. Unsurprisingly, his first recording of the work lasts for less than forty minutes, the second for more than fifty: he was playing fast and slow.

Learning to think is hard for two reasons. First, most difficult situations we face in life are unique, even though the challenges are ubiquitous. Learning how to support our children as they grow up, or how to manage our relations with our parents as they grow old, or how to maintain friendships over the years as the ties that once bound us together loosen apart, are experiences common to many of us; but that does not mean that there are simple, standard solutions. Each version of the problem has its individual complexities which make it relevantly unlike other versions of the problem. That's one reason why books that set out ten rules for success, or seven principles that will lead to happiness, are so obviously misleading: there are no generic answers to life's important questions.

A second reason is that as we grow older, we keep learning; indeed, one might go so far as to say (with Jürgen Habermas) that a characteristic of the human species is our inability not to learn. While all of us think differently when we are fifty to how we thought when we were twenty-five, some of us exhibit this trait more reliably than others. As we learn, so we find better answers to questions that perplexed us when we were younger, and we also discover that some answers we previously accepted are in fact not so compelling. Our judgement improves with use, which means that we need to keep building on its successes and working to reduce its failures. Growing old is unavoidable, but becoming wiser is a choice, requiring time and effort.

One thing that doesn't improve with age is our speed of thought. Quickness is a gift of youth, and we will always celebrate the brilliance and inventiveness of the prodigy. There is no reason to condescend just because we can no longer keep up with the generations that follow on from us. But as Goethe came to understand – having himself been the youthful genius of German culture in the second half of the eighteenth century – the mental agility of clever young minds needs to give way to the patient accumulation of good practice, which over time constitutes a life well lived.

I recommend Kahneman's book. It is beautifully written, erudite and insightful. It is both a critique of our tendency to rely on quick, immediate thoughts, and a pæan to the cultivation of slow, careful, evidence-based reasoning. It is a thorough presentation, using modern psychological research, of the case that Aristotle made more than two thousand years ago, that a lifetime devoted to good thinking is the most reliable route to happiness and the best protection against failure.